Nahum Gat

Methodology for hyperspectral image classification using novel neural network

A novel feed forward neural network is used to classify hyperspectral data from the AVIRIS sensor. The network applies an alternating direction singular value decomposition technique to achieve rapid training times. Very few samples are required for training. 100 percent accurate classification is obtained using test data sets. The methodology combines this rapid training neural network together with data reduction and maximal feature separation techniques such as principal component analysis and simultaneous diagonalization of covariance matrices, for rapid and accurate classification of large hyperspectral images. The results are compared to those of standard statistical classifiers.

Spatial mode filtering of mid-infrared (mid-IR) laser beams with hollow core fiber optics

Measurements characterizing spatial mode filtering of mid-infrared (mid-IR) laser beams using hollow core fiber optics are presented. The mode filtering depends strongly on the fiber diameter, with effective mode filtering demonstrated with bore diameters of d = 200 μm and 300 μm. In addition to mode filtering, beam profile measurements also demonstrate the strong dependence of the mode quality on the fiber coupling conditions. As predicted, optimal coupling is achieved using relatively slow optics that produce focused spots that nearly fill the fiber diameter. Examples of the utility of using hollow fibers for mode-filtering to improve molecular spectroscopy experiments are also discussed.

Subpixel object detection using hyperspectral imaging for search and rescue operations

Time critical search & rescue (s&r) operations often requires the detection of small objects in a vast area. While an airborne search can cover the area, no operational instrumental tools currently exist to actually replace the human operator. By producing the spectral signature of each pixel in a spatial image, multi- and hyper-spectral imaging (HSI) sensors provides a powerful capability for automated detection of subpixel size objects that are otherwise unresolved objects in conventional imagery. This property of HSI naturally lends itself to s&r operations. A lost hiker, skier, life raft adrift in the ocean, downed pilot or small aircraft wreckage targets, can be detected from relatively high altitude based on their unique spectral signatures. Moreover, the spectral information obtained allows the search craft to operate at substantially reduced spatial resolution thereby increasing scene coverage without a significant loss in detection sensitivity. The paper demonstrates the detection of objects as small as 1/10 of an image pixel from a sensor flying at over 6 km altitude. A subpixel object detection algorithm using HSI, based on local image statistics without reliance on spectral libraries is presented. The technique is amenable to fast signal processing and the requisite hardware can be built using inexpensive off the shelf technology. This makes HSI a highly attractive tool for real-time, autonomous instrument-based implementation. It can complement current visual-based s&r operations or emerging synthetic aperture radar sensors that are much more expensive.

Variable Cold Stop for Matching IR Cameras to Multiple f-number Optics

Cameras operating in the thermal infrared (mid-wave and long-wave IR) use a cold stop that is designed to match the exit pupil of the optics and thus avoid parasitic radiation or vignetting. For years, range operators have been using reflective telescopes, usually with photo-documentation film cameras. Along with the need to shift operation into the infrared comes a problem that (i) these telescopes do not have an exit pupil located at the IR camera cold stop, and (ii) most IR cameras have f/2 or f/4 stop, while the telescope is typically f/7 or greater. These mismatches cause a significant deterioration of the system performance and picture quality. A similar need arises when using zoom optics with IR cameras where, as the field of view changes, so does the optics f/#, creating a mismatch with the camera that has a fixed aperture. The OKSI/WSMR team has demonstrated two implementations of a patented continuous variable aperture / cold stop (CVA/CS or VariAp®) for operating IR cameras with different f/# optics. Two systems were built: (1) an optical relay assembly with an external CVA/CS, and (2) a custom 1024×1024 pixel MWIR camera with a built in CVA/CS and the proper relay optics to match the telescope optics to the camera. The first optical relay with the VariAp® is a retrofit for legacy IR cameras for operations with reflective telescopes. The camera with the built-in VariAp® can function with both reflective (using an additional external relay) and refractive (with no additional relay) telescopes. The paper describes the two systems that open new possibilities in IR imaging for various ranges.

True-color night vision (TCNV) fusion system using a VNIR EMCCD and a LWIR microbolometer camera

A fully functional, prototype night vision camera system is described which produces true-color imagery, using a visible/near-infrared (VNIR) color EMCCD camera, fused with the output from a thermal long-wave infrared (LWIR) microbolometer camera. The fusion is performed in a manner that displays the complimentary information from both sources without destroying the true-color information. The system can run in true-color mode in day-light down to about 1/4-moon conditions, below this light level the system can function in a monochrome VNIR/LWIR fusion mode. An embedded processor is used to perform the fusion in real-time at 30 frames/second and produces both digital and analog color video outputs. The system can accommodate a variety of modifications to meet specific user needs, and various additional fusion algorithms can be incorporated making the system a test-bed for real time fusion technology under a variety of conditions.

Chemical detection using the airborne thermal infrared imaging spectrometer (TIRIS)

A methodology is described for an airborne, downlooking, longwave infrared imaging spectrometer based technique for the detection and tracking of plumes of toxic gases. Plumes can be observed in emission or absorption, depending on the thermal contrast between the vapor and the background terrain. While the sensor is currently undergoing laboratory calibration and characterization, a radiative exchange phenomenology model has been developed to predict sensor response and to facilitate the sensor design. An inverse problem model has also been developed to obtain plume parameters based on sensor measurements. These models, the sensors, and ongoing activities are described.

Fiber Optics for Remote Delivery of High Power Pulsed Laser Beams

The work described here is focused on the technology to enable remote fiber optic delivery of high-power, pulsed laser beams for diagnostics used in combustion and flow-field characterization. Fiber delivery is desirable since it is not always practical to locate laser diagnostic equipment in close proximity to the harsh environment associated with propulsion test facilities (e.g., jet or rocket engine testing). In this study both onedimensional hollow core waveguides and photonic bandgap fibers were investigated. Relatively large bore (~ 1000 µm) hollow waveguides were found to be the optimal fiber solution for their ability to deliver relatively high peak power pulses (5 ns duration at energies > 10 mJ/pulse) with an acceptable beam quality (M 2 ~ 10 to 20). Using such waveguides, a Coherent Anti-Stokes Raman Spectroscopy (CARS) system with fiber delivery of the laser beams was fully demonstrated in laboratory experiments. The technology is currently being applied to a field-ready CARS system, which will be initially demonstrated by mapping the temperature of exhaust products in a jet engine plume.

Thermal Infrared Imaging Spectrometer (TIRIS) status report

The TIRIS is a pushbroom long wave infrared imaging spectrometer designed to operate in the 7.5 to 14.0 micrometer spectral region from an airborne platform, using uncooled optics. The focal plane array is a 64 by 20 extrinsic Si:As detector operating at 10 K, providing 64 spectral bands with 0.1 micrometer spectral resolution, and 20 spatial pixels with 3.6 milliradians spatial resolution. A custom linear variable filter mounted over the focal plane acts to suppress near field radiation from the uncooled external optics. This dual- use sensor is developed to demonstrate the detection of plumes of toxic gases and pollutants in a downlooking mode.

Capturing 3D hyperspectral image cubes for dynamic events

Hyperspectral imaging systems capture spectral and spatial information and produce a hyperspectral data cube, or ‘hypercube,’ from the data. These hypercubes consist of 2D spatial images with spectra at each pixel. Hyperspectral imaging is used in many applications, including agricultural health monitoring from aircraft or satellites, mineral detection and identification, food processing, and medical diagnostics. Typical hyperspectral systems use some form of scanning—tunable filters1 or laser illumination,2 for example—to build up a hypercube over multiple camera frames, and are therefore unable to temporally resolve dynamic events. Some systems image only a single spatial slice of the cube at one time and must scan over the spatial dimension using platform motion or a rotating mirror.3 For applications that image dynamic events (those that change within the scanning time), these methods are not sufficient to accurately capture the 4D data content (2D spatial C spectral C temporal). We have developed a new device, the 4D imaging spectrometer4 (4DIS), capable of the high spectral and temporal resolution required for quickly evolving events. The sensor obtains an image cube with each camera frame, enabling capture and analysis of ‘hypervideo’ sequences. The fundamental concept of the 4DIS is shown in Figure 1. The lens forms an image onto a 2D array of fiber optics. At the opposite end, the fibers are rearranged to form a linear column. In other words, the 2D spatial content is collapsed to 1D with the 2D information preserved. The column of fibers defines the input slit of an imaging spectrograph (e.g., Offner), from which data is captured using a scientific 2D imaging camera. The output from this camera contains the spatial and spectral information that can be remapped to a 3D image cube in post-processing. We have developed 4DIS sensors in several spectral regions: ultraviolet, visible to near-infrared (VNIR), short-wave infrared, mid-wave infrared, and long-wave Figure 1. Fundamental concept of the 4D imaging spectrometer (4DIS) sensor: a lens focuses the image onto a 2D fiber array feeding to an imaging spectrometer. The fiber reformats the 2D array of pixels (x, y) into a 1D line (x y), which is remapped at the end to produce the 3D image cube. : Wavelength.

Fiber Delivery of mid-IR lasers

Fiber optics for the visible to near infrared (NIR) wavelength regimes (i.e. = 0.42 {mu}m) have proven to be extremely useful for a myriad of applications such as telecommunications, illumination, and sensors because they enable convenient, compact, and remote delivery of laser beams. Similarly, there is a need for fiber optics operating at longer wavelengths. For example, systems operating in the mid-IR regime (i.e., = 314 {mu}m) are being developed to detect trace molecular species with far-reaching applications, such as detecting explosives on surfaces, pollutants in the environment, and biomarkers in the breath of a patient. Furthermore, with the increasing availability of quantum cascade lasers (QCLs) which are semiconductor lasers that operate in the mid-IR regime additional uses are rapidly being developed. Here, we describe the development of hollow-core fibers for delivery of high-quality mid-IR laser beams across a broad spectral range.